1. Field of the Invention
This invention relates to a semiconductor device, in particular a semiconductor device having a vertical MOS (Metal-Oxide-Semiconductor) gate structure, which requires a high-speed switching property, and a method of manufacturing the same.
2. Description of the Related Art
In conventional art, a trench-gate structures formed by using a trench formed at a principal surface of a semiconductor device is applied to semiconductor devices, such as IGBT (Insulated Gate Bipolar Transistor) and MOSFET (Field Effect Transistor), and is an advantageous structure in, in particular, use for electricity.
For example, MOSFETS having a trench-gate structure have a high switching speed, a large current capacity, and a capacity of breakdown voltage of tens to hundreds, and thus have come into wide use. For example, they are used as a switching power supply for Pocket mobile communications apparatus and personal computers.
In particular, with increase in speed and efficiency of power supply systems, reducing an on-resistance and a gate-drain feedback capacitance of devices is regarded as important increasingly, in power MOSFETs used for DC—DC converters. FIG. 1 shows a cross-sectional structure of a conventional trench-gate type MOSFET (for example, please refer to Jpn. Pat. Appln. KOKAI Pub. No. 5-7002).
However, in the conventional trench-gate type MOSFET shown in FIG. 1, a gate electrode 101 has a large facing area on an n-type semiconductor layer (drain layer), and thus the MOSFET has a large gate-drain capacitance. Therefore, it has a problem that a Miller charging period at the time of turning on and off the device is long, and that a high-speed switching cannot be achieved. Thus, to achieve high speed (high frequency) and high efficiency of power supply systems, it is urgently necessary to reduce the on-resistance and the gate-drain capacitance.